This invention relates to an article made of a nickel-base superalloy, and, more particularly, to the protection of the surface of such an article with a thermal barrier coating.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the gas turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1800-2100xc2x0 F.
Many approaches have been used to increase the operating temperature limit and service lives of the turbine blades and vanes to their current levels. For example, the composition and processing of the base materials themselves have been improved. Cooling techniques are used, as for example providing the component with internal cooling passages through which cooling air is flowed.
In another approach, the surfaces of the turbine blades and vanes are coated with thermal barrier coatings (TBCs). The TBCs typically include an aluminum-containing bond coat that contacts the substrate, and a ceramic layer overlying the bond coat. The bond coat protects the articles against the oxidative and corrosive effects of the combustion gas, and the ceramic layer provides thermal insulation. The turbine blades and turbine vanes are thereby able to run cooler and are more resistant to environmental attack. While TBCs are operable, it has been observed that their performance is sometimes inconsistent as a result of premature cracking and other failure mechanisms. Accordingly, there is a need for an improved approach to the preparation of nickel-base superalloys with thermal barrier coatings. The present invention fulfills this need, and further provides related advantages.
The present invention provides a technique for improving the performance of a thermal barrier coating (TBC) on a nickel-base superalloy. This improvement is achieved while using the known techniques for preparing the TBC, with a relatively inexpensive process addition. The present approach may be used with a variety of types of TBCs, without losing the otherwise beneficial aspects of these TBCs.
A method for preparing a nickel-base superalloy protected by a thermal barrier coating comprises the steps of furnishing a substrate made of a nickel-base superalloy, thereafter depositing a bond coat layer overlying and contacting the substrate, thereafter depositing a ceramic layer overlying and contacting the bond coat layer, thereby forming a coated substrate, thereafter placing the coated substrate into a heating apparatus operating with an oxidizing atmosphere, and thereafter heating the coated substrate in the heating apparatus to a temperature of from about 1850xc2x0 F. to about 2100xc2x0 F., for a time of at least about 30 minutes. The heating time is typically from about 30 minutes to about 12 hours. The article is thereafter placed into service.
In another embodiment, a method for preparing a nickel-base superalloy protected by a thermal barrier coating comprises the steps of furnishing a coated substrate comprising a substrate made of a nickel-base superalloy, a bond coat layer overlying and contacting the substrate, and a ceramic layer overlying and contacting the bond coat layer. The method further includes thereafter placing the coated substrate into a heating apparatus, and heating the coated substrate in the heating apparatus to a temperature sufficient to grow a layer comprising primarily, preferably entirely, alpha alumina on the bond coat layer, between the bond coat layer and the ceramic layer.
In each approach the bond coat layer may be a diffusion aluminide or an overlay coating. The ceramic layer preferably comprises yttria-stabilized zirconia, although other types of ceramics may be used. The heating apparatus may be an air furnace, a furnace operating with a partial pressure of oxygen, or a furnace using a partial vacuum.
The controlled furnace heating in an oxidizing atmosphere causes a thermally grown oxide to form on the surface of the bond coat layer, between the bond coat layer and the ceramic layer. The furnace preferably provides a partial pressure of oxygen to ensure a relatively slow, uniform growth of the thermally grown alumina. The thermally grown oxide is the stable form of alumina (aluminum oxide), alpha (xcex1) alumina, rather than one of the many other forms of alumina. This alpha alumina thermally grown oxide is primarily of a uniform alpha alumina type, rather than a mixture of types that may result from growth at other temperatures or by other processing methods. By contrast, prior approaches for forming the thermally grown alumina have relied upon heating during the deposition of the bond coat or the deposition of the ceramic layer, or upon heating during service of the engine. All of these operations involve heating that produces a thermally grown oxide of undefined thickness and type. The present approach provides a carefully controlled heating that results in a thermally grown oxide of uniform thickness and crystallographic structure. The present approach does not exclude heating during deposition or during service, but adds a controlled heating step after the ceramic layer is deposited but before the TBC-coated article is placed into service. The result is less variability in the final coated article that is placed into service, as well as improved service life. Further, the strength of the interface between the alpha alumina grown by the present approach and the ceramic layer of the thermal barrier coating is expected to be higher than found for other types of alumina.
The present approach results in a more uniform and higher-quality TBC-coated nickel-base superalloy article, with only the addition of a heat treatment after the ceramic layer is coated onto the article. Preferably, the alumina scale on the bond coat layer is uniformly alpha alumina. The use of this single form of alumina ensures that there will be no constrained phase transitions among a mixture of forms of alumina during the service life, leading to improved long-term stability of the TBC. Constrained phase transformations result in residual stresses within the layer, which tend to induce spallation failure. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.